The warming of swimming pools is one of the most practical and successful applications of solar energy. For this use of solar energy, the often intermittent presence of direct sunlight does not create difficulties or inconvenience because swimming pools are sizable thermal reservoirs. One or two days of cloudy weather does not usually cause the pool temperature to become uncomfortable. The main obstacle to a more widespread adoption of solar pool heating is the initial cost of the equipment. The primary factor determining the expense involved with most solar heating and power systems is that they require a large amount of material in order to present a large surface area for insolation (exposure to sunlight).
A typical solar panel consists of water passages bonded to a sunlight absorption surface that is covered by transparent glazing. The technology is not complicated but it usually requires a lot of copper and glass. A disadvantage of glazed solar collectors is that they may be heavily damaged by hail storms. More rugged, hail resistant, solar panels can be made, but at greater expensive.
For swimming pool applications, solar panels without glazing are very common. These collectors consist only of flexible black plastic or rubber panels with internal water passages. This less expensive kind of solar collector will have somewhat lower performance than a glazed collector particularly during windy weather or when the ambient air is cold. Plastic and rubber are generally not durable when exposed to sunlight especially when compared to a metal surface. The sizing rule of thumb commonly used by solar panel vendors recommends a collector panel area of 50 to 120% of the swimming pool free surface area. Purchasing enough plastic panels to meet this criterion for a moderately sized pool costs at least several hundred to a thousand dollars. For a 600 ft2 pool surface, buying 50% of this area in solar panels costs $1200, and purchasing 120% of the pool area would cost $2830 according to a current swimming pool supply catalog. A fixed solar panel (without a sun tracking mechanism) has to be oriented properly to maximize the amount of insolation it receives. Accordingly, such a panel may be at peak effectiveness during only a few hours of a day. Moreover, in some cases there may not be a suitable expanse of roof surface in the proper orientation for which to mount a solar collector. Another disadvantage of roof-mounted solar panels is the necessity of taking them down and then reinstalling them whenever the roof shingles are replaced. Roof-mounted panels will remove some of the load on a house's cooling system by keeping portions of the roof surface from getting hot. However, in most cases solar panels will only cover a small fraction of a roof surface.
Another technique for warming a swimming pool is to use a translucent plastic pool surface cover with encapsulated air pockets. Such solar blankets or “bubble wrap” covers help reduce evaporation and increase solar gain, but they are inconvenient to stow when the pool is to be used. Pool cover reels can make stowage much easier, but these reels typically cost hundreds of dollars. Some swimming pools have irregular shapes which make a pool cover and reel particularly impractical. Furthermore, the bubble covers tend to degrade from sunlight exposure. In addition, a pool cover cannot contribute heat while people are swimming in the pool. Accordingly, frequent swimming during the afternoon hours could significantly detract from the benefits of a pool cover.
Fuel-burning pool heaters are still the most common type of pool heater in use. Fuel-burning heaters tend to have a much greater heat output rate than solar systems. For example, natural gas pool heaters typically have heating outputs in the range of 100,000 to 400,000 BTU/hr. To put this quantity in better perspective, a 50-gallon domestic water heater produces about 40,000 BTU/hr and the typical output of a residential furnace is about 50,000 BTU/hr. Accordingly, such combustion pool heaters are often capable of raising the temperature of the pool water 5.6 to 11° C. (10 to 20° F.) within the course of a day. However, the cost associated with such a rapid warm-up makes this practice seem extravagant. Moreover, due to the finite supply, fossil fuels will become increasingly scarce and expensive. Hydrocarbon-burning heaters release carbon dioxide into the atmosphere contributing to harmful global climate change. Unlike roof-mounted solar collectors, fuel-burning heaters do not remove any of the load on a house's cooling system. Another shortcoming of natural gas swimming pool heaters is how quickly they rust and corrode. This can be attributed to fact that the chemical processes of corrosion and rusting will be accelerated with an increase in temperature. The metal parts within a combustion pool heater are exposed to a much higher level of temperatures than what is encountered by an ordinary flat plate solar collector. Combustion heaters must have safeguard devices such as flame detectors and fusible links in order to prevent the hazards of fire, explosion or incomplete combustion. Such failsafe devices are commonplace for fuel-burning appliances but the necessary additional parts add to the expense and service requirements of a heater.
In addition to preventing an attic from accumulating excess moisture, attic ventilation fans are highly effective in reducing the temperature within an attic space. By exhausting hot air and bringing in fresh air, an attic ventilation fan removes some of the overall cooling load on the house by substantially reducing the temperature difference that drives the heat transfer between the attic and the living space. Heat is not only conducted from the attic to the living space below through the ceiling partition, but also through the walls of the air conditioning ducts within the attic. Air conditioning ducts generally have a much thinner layer of insulation (R value range of 2 to 4) than the layer over the ceiling (R value range of 11 to 38). For the cool air supply ducts, the temperature difference between the attic air and the air within the duct is 8.3 to 11.1° C. (15-20° F.) larger than the temperature difference between the attic and living space. Even a well-insulated attic often has gaps or breaks in the insulation layer which would allow a significant amount of heat to ‘leak’ through to the living space. For example, a 2-ft ft by 4-ft non-insulated attic access door would allow 472 W (1613 BTU/hr) of heating to transfer to the living space from a 140° F. attic. If ventilation was used to bring the attic temperature down to 100° F., then this would reduce the heat transfer rate to 173 W (589 BTU/hr.). Another benefit of attic ventilation is increased service life for asphalt shingles due to the decrease in temperature to which they are exposed. In addition, the discomfort and hazard of working in an attic during the warm part of the year is considerably reduced. Keeping the attic space from reaching high temperatures also allows items to be stored in the attic that would otherwise be damaged by the higher temperatures encountered within an unventilated attic. However, the substantial quantity of heat carried away by an attic ventilation system is discarded rather than utilized.
An example of early effort to exploit attic heat accumulation for warming a swimming pool is R. David Burns' installation of finned copper tubes inside his attic for the purpose of circulating pool water through them. However, the success of this approach was severely limited due to an insufficient rate of convective heat transfer because no means of forced air movement was provided. The convective heat flux (heat transfer rate per unit area) is equal to the temperature difference between a surface and the fluid surrounding it times the convection coefficient. For air movement due only to thermally-induced buoyancy effects (free convection), the convection heat transfer coefficient (or film coefficient) falls in the range of 5 to 25 W/(m2·K). For forced convection where the air is propelled to move, the convection heat transfer coefficient will range between 25 and 250 W/(m2·K). Accordingly, the rate of convective heat transfer is strongly influenced by the velocity of the fluid (gas or liquid) wetting a heat exchanger surface. In addition to the influence of fluid velocity, effective heat transfer between a liquid and a gas requires much more surface area on the gas side of the heat exchanger. Edward G. Palmer introduced an attic-mounted compact air-to-water heat exchanger with forced convection provided by a fan placed at the face of the heat exchanger. The implementation of forced convection raised the heat transfer rate by a factor of 4 to 5 as compared to the Burns installation and also reduced the amount of heat exchanger material required. This development by Edward G. Palmer was patented and commercially developed as the SolarAttic brand model PCS1 and the later model PCS2.
The SolarAttic unit introduces water plumbing into the attic space generally as a retrofit. Such an installation can be done successfully, but great care must be taken to prevent and contain water leaks that would cause substantial damage to the house below. The SolarAttic product does include features such as a water-tight pan and a float sensor to minimize the risk of a leak, but these features inevitably add to the cost of the product. Additionally, since the plumbing is routed through unconditioned space in the attic, measures must be taken to prevent a freeze-induced rupture of the heat exchanger and its plumbing. The prospect of a damaging leak may be much more of a serious issue as a perceived risk by potential consumers rather than an actual risk. Potential customers may be particularly intimidated by SolarAttic's practice of sending the entire flow from a pool pump (up to 3.47 liters per second or 55 gallons per minute) to an attic-mounted heat exchanger. Moreover, the necessary skill to achieve a leak-free installation on the first attempt would tend to exclude most consumers from being able to save money by installing the SolarAttic product themselves rather than hiring a professional. The chances of a damaging leak would depend more on the skill and care undertaken by the installer rather than the degree of manufacturing quality of the attic-mounted unit. If an attic space is fairly confined, it may be difficult or impossible to install and/or maintain the SolarAttic product. If a roof design does not include gables, then it can be difficult to run water lines into the tight space at the juncture where the roof and ceiling framework meet. In many houses, there are wooden structural members at this crowded juncture that would tend to obstruct water lines installed in straight path from the eaves to the attic interior. For warm climates such as the Southern United States, no heating of the swimming pool water is required during much of the summer; however the attic space temperature would still need to be minimized to reduce the cooling load on the house. In other words, there will be times that pool heating is not needed, but attic ventilation is needed more than at any other time of the year. In this situation, the SolarAttic product can not reduce the cooling load on the house without overheating the swimming pool. Accordingly, some form of attic ventilation equipment is required in addition to the SolarAttic installation. Furthermore, this ventilation equipment should be disabled and closed-off for the SolarAttic unit to function properly. The SolarAttic PSC2 sells for over $3000 making it more expensive than most fuel-burning heaters and solar heating systems. Moreover, a temporary installation for sales demonstration purposes is highly impractical. Accordingly, a buyer of the SolarAttic product would not have the opportunity to test its effectiveness at his or her home before making the purchase. The heat output of the SolarAttic product will depend on the particular characteristics of a house and the conditions it is exposed to.
There is a need for improved systems for heating swimming pools and ventilating attics.